Laser processing apparatus and method

ABSTRACT

A controller outputs a first event signal having a periodical waveform and a second event signal having a periodical waveform synchronizing with the first event signal. A first laser source radiates a first pulse laser beam having a wavelength in an ultraviolet range, synchronously with the first event signal. A second laser source radiates a second pulse laser beam having a wavelength in the ultraviolet range, synchronously with the second event signal. A converging optical system converges the first and second pulse laser beams at the same point. A holder holds a workpiece at a position where a pulse laser beam converged by the converging optical system is applied.

This application is based on Japanese Patent Application 2000-173244,filed on Jun. 9, 2000, the entire contents of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

a) Field of the Invention

The present invention relates to a laser processing apparatus andmethod, and more particularly to a laser processing apparatus and methodfor applying a pulse laser beam having a wavelength in an ultravioletrange to a workpiece and forming a hole in or through the workpiece.

b) Description of the Related Art

A conventional laser processing method will be described by taking as anexample a method of forming a hole in or through a multi-layer wiringsubstrate. An infrared pulse laser beam radiated from a carbon dioxidegas laser oscillator is converged at a resin layer of a multi-layerwiring substrate. Organic substance in the region applied with the laserbeam is thermally decomposed and a hole is formed in this region. Withthis method, a through hole 100 to 200 μm in diameter can be formedthrough a resin layer about 40 to 80 μm thick. A carbon dioxide gaslaser oscillator can radiate a pulse laser beam having a high energy perone pulse. This pulse laser beam can form a through hole, for example,by three shots.

Holes having shorter diameters are desired to be formed in a multi-layerwiring substrate of a semiconductor integrated circuit device which isimplemented at a higher integration density. A lower limit of thediameter of a hole is about five times the wavelength of a laser beamused. If a carbon dioxide laser is used, the lower limit of a hole isabout 50 μm. It is practically difficult to form a hole having adiameter smaller than 50 to 60 μm by using a carbon dioxide gas laser.

If a laser beam having a wavelength in the ultraviolet range is used, ahole having a smaller diameter can be formed. It is difficult, however,to generate a laser beam having a wavelength in the ultraviolet rangeand a large power. If a laser beam having a small power is used forprocessing a multi-layer wiring substrate, a process time prolongs andproductivity lowers.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a laser processingapparatus and method capable of shortening a process time by using alaser beam having a wavelength in the ultraviolet range.

According to one aspect of the present invention, there is provided alaser processing apparatus comprising: controller for outputting a firstevent signal having a periodical waveform and a second event signalhaving a periodical waveform synchronizing with the first event signal;a first laser source for radiating a first pulse laser beam having awavelength in an ultraviolet range, synchronously with the first eventsignal; a second laser source for radiating a second pulse laser beamhaving a wavelength in the ultraviolet range, synchronously with thesecond event signal; a converging optical system for converging thefirst and second pulse laser beams at a same point; and holder forholding a workpiece at a position where a pulse laser beam converged bythe converging optical system is applied.

According to another aspect of the present invention, there is provideda laser processing method comprising the steps of: radiating a firstpulse laser beam from a first laser source, the first pulse laser beamhaving a wavelength in an ultraviolet range; radiating a second pulselaser beam from a second laser source synchronously with the first pulselaser beam, the second pulse laser beam having a wavelength in theultraviolet range; and applying the first and second pulse laser beamsto a same processing area of a workpiece to form a hole in the sameprocessing area.

When the pulses of the first and second pulse laser beams arealternately applied to the same point of a workpiece, a process speedcan be approximately doubled. When the pulses of the first and secondpulse laser beams are overlapped, the energy per one pulse can beincreased so that a workpiece can be processed which requires a largeenergy for forming a hole.

According to another aspect of the present invention, there is provideda laser processing method comprising the steps of: preparing a workpiecehaving a first layer and a second layer formed under the first layer,wherein a hole can be formed in the first layer by applying anultraviolet pulse laser beam having a first energy per one pulse, and ahole can be formed in the second layer by applying an ultraviolet pulselaser beam having not the first energy per one pulse but a second energyper one pulse higher than the first energy; applying a first pulse laserbeam and a second pulse laser beam to the first layer in a processingarea thereof under a timing condition that pulses of the first andsecond pulse laser beams are alternately applied to the first layer, toform a first hole in the first layer and expose a partial surface of thesecond layer under the first layer, the first pulse laser beam beingradiated from a first laser source and having a wavelength in anultraviolet range, and the second pulse laser beam being radiated from asecond laser source and having a wavelength in the ultraviolet range;and applying the first and second pulse laser beams to the second layerexposed on a bottom of the first hole under a timing condition thatpulses of the first and second pulse laser beams are at least partiallyoverlapped, to form a second hole in the second layer, the first pulselaser beam being radiated from the first laser source and having thewavelength in the ultraviolet range, and the second pulse laser beambeing radiated from the second laser source and having the wavelength inthe ultraviolet range.

By chanting the timing conditions of the first and second pulse laserbeams, the first and second layers can be processed continuously.

According to another aspect of the present invention, there is provideda laser processing apparatus comprising: controller for outputting afirst event signal having a periodical waveform and a second eventsignal having a periodical waveform synchronizing with the first eventsignal; a first laser source for radiating a first pulse laser beamhaving a wavelength in an infrared or visual range, synchronously withthe first event signal; a second laser source for radiating a secondpulse laser beam having a wavelength in the infrared or visual range,synchronously with the second event signal; an optical propagationsystem for changing an optical axis of at least one of the first andsecond laser beams so as to make the first and second pulse laser beamspropagate along a same optical axis; a non-linear optical component forgenerating a harmonic wave having a wavelength in an ultraviolet range,from the first and second pulse laser beams made to have the sameoptical axis by the optical propagation system; a converging opticalsystem for converging the harmonic wave; and holder for holding aworkpiece at a position where the harmonic wave converged by theconverging optical system is applied.

As the pulses of the first and second pulse laser beams alternatelyreach the non-linear optical component, a harmonic wave is generatedhaving a repetition frequency twice as high as the repetition frequencyof each of the input pulse laser beams. A process time can therefore beshortened. When the pulses of the first and second pulse laser beams areoverlapped and become incident upon the non-linear optical component,the energy per one pulse of the harmonic wave increases so that aworkpiece can be processed which requires a large energy for forming ahole.

As above, by combining the pulse laser beams radiated from the two lasersources to have a predetermined phase difference, the hole forming timecan be shortened. By overlapping the pulses of the first and secondpulse laser beams, the energy per one pulse can be increased. Even if asufficient energy per one pulse cannot be obtained by one laser source,a sufficient energy can be obtained by using two laser sources. A holecan be formed in a workpiece even if it requires a large energy per onepulse.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a laser processing apparatus accordingto an embodiment of the invention.

FIG. 2 is a timing chart showing the operation of a first control modeof the laser processing apparatus according to the embodiment.

FIG. 3 is a timing chart showing the operation of a second control modeof the laser processing apparatus according to the embodiment.

FIG. 4 is a graph showing an example of the output characteristics of athird harmonic wave of an Nd:YAG laser.

FIG. 5 is a cross sectional view of a multi-layer wiring substrate.

FIG. 6 is a block diagram showing a laser processing apparatus accordingto another embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram showing a laser processing apparatus accordingto an embodiment of the invention. First and second laser sources 1 and2 radiate pulse laser beams pl₁ and pl₂ having a wavelength in theultraviolet range, synchronously with event signals sig₁ and sig₂. Thefirst and second laser sources 1 and 2 each include, for example, anNd:YAG laser oscillator and non-linear optical components. The pulselaser beams ply and pl₂ each are, for example, a third harmonic wave(355 nm in wavelength) of a pulse laser beam radiated from an Nd:YAGlaser oscillator. The pulse laser beams pl₁ and pl₂ are linearlypolarized, respectively in vertical and horizontal directions.

The pulse laser beam pl₁ radiated from the first laser source 1 isreflected by a turn-around mirror 5 and becomes incident upon the frontsurface of a polarizer 6 at an incidence angle of 45°. The pulse laserbeam pl₂ radiated from the second laser source 2 is incident upon theback surface of the polarizer 6 at an incidence angle of 45°. Thepolarizer 6 reflects the pulse laser beam pl₁ which was linearlypolarized in the vertical direction, and transmits the pulse laser beampl₂ which was linearly polarized in the horizontal direction.

The pulse laser beams pl₁ and pl₂ are combined on the same optical axisby the polarizer 6 to form a pulse laser beam pl₃. The pulse laser beampl₃ is reflected by a turn-around mirror 9. The reflected pulse laserbeam pl₄ becomes incident upon a galvano scanner 10. The galvano scannerscans the optical axis of the pulse laser beam pl₄ in a two-dimensionaldirection in response to a command signal sig₀.

The pulse laser beam passed through the galvano scanner 10 is convergedby a converging lens 11 to form a pulse laser beam pl₅. For example, theconverging lens 11 is an fθ lens. A workpiece 20 is held by a holder 12at a converging position of the pulse laser beam pl₅.

A control unit 13 supplies the first and second laser sources 1 and 2with the event signals sign and sig₂ having a periodical waveform. Thecontrol unit 13 selects one of first and second control modes and cansupply the event signals sig₁ and sig₂ having a phase differencespecific to each control mode. The control unit 13 also supplies thegalvano scanner 10 with the control signal sig₀.

Next, with reference to FIGS. 2 and 3, timings of the pulse laser beamsused by the laser processing apparatus shown in FIG. 1 will bedescribed.

FIG. 2 is a timing chart of the first control mode. The event signalssig₁ and sig₂ are pulse signals having the same frequency andsynchronized with each other. The phase of the event signal sig₂ lags by180 degrees from the phase of the event signal sig₁. The pulse laserbeam pl₁ is synchronous with the event signal sig₁, whereas the pulselaser beam pl₂ is synchronous with the event signal sig₂. The pulselaser beam pl₂ lags therefore by 180°0 in phase from the pulse laserbeam pl₁. The pulse repetition frequencies of the pulse laser beams pl₃to pl₅ formed through combination of the pulse laser beams pl₁ and pl₂are twice the frequency of the event signals sig₁ and sig₂.

FIG. 4 is a graph showing an example of the output characteristics of athird harmonic wave of each of the first and second laser sources 1 and2 using Nd:YAG laser oscillators. The abscissa represents a pulserepetition frequency in the unit of “kHz”, and the ordinate represents alaser output in the unit of “W”. At the repetition frequency of about 5kHz, the laser output takes a maximum value. In the repetition frequencyrange not lower than 5 kHz, the laser output gradually lowers as therepetition frequency becomes high. This tendency is not limited only toan Nd:YAG laser oscillator, but other solid state lasers have similartendency.

In order to form a hole in or through a resin film, an energy densityper one pulse of a pulse laser beam is generally required to have somethreshold value or higher. For example, if a hole is to be formed in anepoxy resin film, the energy density per one pulse is required to haveabout 1 J/cm² or higher. An energy per one pulse necessary for forming ahole is determined from the area of the hole. The energy per one pulseis given by p/f [J], where P [W] is an output of the pulse laser beamand f [Hz] is a pulse repetition frequency. The range where the energyP/f per one pulse takes the necessary threshold value or higher can bedetermined from the output characteristics shown in FIG. 4. If the lasersources 1 and 2 are operated in this range, a hole can be formed in aresin film.

The repetition frequency of the pulse laser beam pl₅ applied to theworkpiece 20 is 10 kHz which is a twofold of the frequency of the eventsignals sig₁ and sig₂. The hole forming time can be shortened by about ½as compared to using one laser oscillator.

FIG. 3 is a timing chart of the second control mode. In the firstcontrol mode shown in FIG. 2, the phase of the event signal sig₂ lags by180° from the phase of the event signal sig₁. In the second controlmode, the phase lag is small. Accordingly, the pulse laser beams pl₁ andpl₂ partially overlap to form the pulse laser beams pl₃ to pl₅ formedthrough combination of the pulse laser beams pl₁ and pl₂. The width ofeach pulse is hence broadened and the energy per one pulse is doubled.The phases of the event signals sig₁ and sig₂ may be set equal tocompletely superpose each pulse of the pulse laser beam pl₁ upon eachpulse of the pulse laser beam pl₂. In this case, the pulse width doesnot broaden but the peak power is approximately doubled.

In order to form a hole in a copper foil, the energy density per onepulse is generally required to be about 10 J/cm² or higher. If thediameter of a hole is 100 μm, the energy per one pulse is required to beabout 7.9×10⁻⁴ J or higher. In the first control mode shown in FIG. 2,it is difficult to set the energy per one pulse to about 7.9×10⁻⁴ J orhigher. By partially overlapping the two pulse laser beams as shown inFIG. 3, the energy per one pulse necessary for forming a hole in acopper foil can be obtained.

Even if the energy per one pulse is insufficient, the necessary energyper one pulse may be obtained by converging the laser pulse and reducingthe beam diameter. However, in this case, since the laser diameter issmall, it is necessary to move the application position of the laserbeam in order to form a hole having a desired size. For example,trepanning or spiral working becomes necessary. As in this embodiment,by increasing the energy per one pulse, a hole having a diameter ofabout 100 μm can be formed without trepanning or the like.

For example, if the repetition frequency is set to 10 kHz, the output ofone laser source is about 4 W as determined from FIG. 4. The power ofeach of the laser beams pl₃ to pl₅ shown in FIG. 3 is therefore 8 W. Theenergy per one pulse is 8×10⁻⁴ J. Although one laser source is difficultto form a hole in a copper foil, the energy per one pulse can be madesufficiently large for forming a hole in a copper foil by using twolaser sources and superposing pulses.

The pulse width and peak intensity of the laser beams pl₃ to pl₅ shownin FIG. 3 depend on the phase difference between the pulse laser beamspl₁ and pl₂. By adjusting the phase difference between the event signalssig₁ and sig₂, the pulse width and peak intensity of the pulse laserbeams pl₃ to pl₅ can be controlled with ease.

FIG. 5 is a cross sectional view of a multi-layer wiring substrate. Apackage board 22 is mounted on the surface of a mother board 21. Asemiconductor integrated circuit chip 23 is mounted on the package board22. The mother board 21 and package board 22 are made of epoxy resinwhich contains glass cloth.

Copper wiring layers 25 are formed embedded in the mother board 21. Avia hole 26 extends from the surface of the mother board 21 to thecopper wiring layer 25. A through hole 27 is formed through the motherboard 21. Copper is filled in the via hole 26 and through hole 27.Similarly, a copper wiring layer 28 and a via hole 29 are formed in thepackage board 22. The via holes 26 and 29 and through hole 27 are formedby using the laser processing apparatus shown in FIG. 1. The laserprocessing is executed for separate mother board 21 and package board 22before the latter 22 is mounted on the former 21.

The via holes 26 and 29 are formed in the first control mode shown inFIG. 2. In this case, the energy per one pulse of the pulse laser beampl₅ is sufficiently large for forming a hole in the resin layer.However, since the energy is insufficient for forming a hole in thecopper wiring layer, the copper wiring layer 25 is left unetched on thebottom of the via hole.

In order to form the through hole 27, after a hole is formed through theresin layer in the first control mode, another hole is formed throughthe copper wiring layer in the second control mode shown in FIG. 3. Inthis latter case, the energy per one pulse of the pulse laser beam pl₅is sufficiently large for forming a hole in the copper foil layer. Thethrough hole 27 can be formed in this manner by alternately repeatingthe laser processing in the first and second control modes.

If a hole is to be formed through a copper foil layer formed on thesurface of a resin substrate, the hole is formed through the copper foillayer first in the second control mode. This laser processing can bestopped automatically when the hole is formed through the copper foillayer, by setting beforehand the number of pulses to be applied, inaccordance with the thickness of the copper foil layer. After the holeis formed through the copper foil layer, then the mode is switched tothe first control mode and another hole is formed through the resinlayer. The number of pulses applied during the laser processing in thefirst control mode is also set beforehand.

In this embodiment, a third harmonic wave of an Nd:YAG laser is used asthe pulse laser beam having a wavelength in the ultraviolet range. Otherlaser beams may also be used. For example, a fourth or fifth harmonicwave of an Nd:YAG laser may be used, and a YLF laser or YVO₄ laser maybe used instead of an Nd:YAG laser. A fundamental wave of a KrF excimerlaser or XeCl excimer laser may also be used.

Also in this embodiment, the pulse laser beam pl₁ radiated from thefirst laser source 1 and the pulse laser beam pl₂ radiated from thesecond laser source 2 are propagated along the same optical axis andconverged at a working position of a workpiece. It is not necessarilyrequired to propagate the pulse laser beams pl₁ and pl₂ along the sameoptical axis. For example, the first and second pulse laser beams pl₁and pl₂ may be propagated along different optical axes which are crossedat the working position of a workpiece.

Next, with reference to FIG. 6, another embodiment of the invention willbe described. In the first embodiment, third harmonic waves of the twoNd:YAG lasers are combined, whereas in the second embodiment,fundamental waves are combined and then the third harmonic wave isformed. The fundamental structure of a laser processing apparatus shownin FIG. 6 is similar to that of the laser processing apparatus shown inFIG. 1. Only different points between the two apparatus will bedescribed.

As shown in FIG. 6, first and second laser sources 1 and 2 radiate pulselaser beams pl₁ and pl₂ having a wavelength in the infrared or visualrange. A non-linear optical component 15 is disposed on the optical axisof a pulse laser beam pl₃ formed through combination of the two pulselaser beams pl₁ and pl₂. The non-linear optical component 15 generates aharmonic wave, e.g., third harmonic wave, of the pulse laser beam pl₃.The non-linear optical component 15 may be disposed anywhere along theoptical path of the pulse laser beam from a polarizer 6 to a workpiece20.

The present invention has been described in connection with thepreferred embodiments. The invention is not limited only to the aboveembodiments. It is apparent that various modifications, improvements,combinations, and the like can be made by those skilled in the art.

What is claimed is:
 1. A laser processing apparatus comprising: acontroller for outputting a first event signal having a periodicalwaveform and a second event signal having a periodical waveformsynchronizing with the first event signal; a first laser source forradiating a first pulse laser beam having a wavelength in an ultravioletrange, synchronously with the first event signal; a second laser sourcefor radiating a second laser pulse beam having a wavelength in theultraviolet range, synchronously with the second event signal; aconverging optical system for converging the first and second pulselaser beams at a same point; and holder for holding a workpiece at aposition where a pulse laser beam converged by said converging opticalsystem is applied.
 2. A laser processing apparatus according to claim 1,wherein said converging optical system changes at least one of the firstand second pulse laser beams and converges the first and second pulselaser beams in such a manner that the first and second pulse laser beamsare propagated along a same optical axis.
 3. A laser processingapparatus according to claim 1, wherein said controller provides a firstcontrol mode and a second control mode, outputs the first and secondevent signals in the first control mode in such a manner that pulses ofthe first and second pulse laser beams are alternately applied to aprocessing position of the workpiece, and outputs the first and secondevent signals in the second control mode in such a manner that pulses ofthe first and second pulse laser beams are at least partially overlappedand applied to the processing position.
 4. A laser processing apparatusaccording to claim 3, wherein said controller can control in the secondcontrol mode an overlap degree between the pulses of the first andsecond pulse laser beams.
 5. A laser processing apparatus according toclaim 1, wherein the first event signal and the second event signal havea same frequency.
 6. A laser processing apparatus according to claim 1,wherein in the second control mode, the first and second pulse laserbeams are partially overlapped so that a pulse width of an overlappedpulse laser beam is broader than that of each of the first pulse laserbeam and the second pulse laser beam.
 7. A laser processing apparatuscomprising: controller for outputting a first event signal having aperiodical waveform and a second event signal having a periodicalwaveform synchronizing with the first event signal; a first laser sourcefor radiating a first pulse laser beam having a wavelength in aninfrared or visual range, synchronously with the first event signal; asecond laser source for radiating a second laser pulse beam having awavelength in the infrared or visual range, synchronously with thesecond event signal; an optical propagation system for changing anoptical axis of at least one of the first and second laser beams so asto make the first and second pulse laser beams propagate along a sameaxis; a non-linear optical component for generating a harmonic wavehaving a wavelength in an ultraviolet range, from the first and secondpulse laser beams made to have the same optical axis by said opticalpropagation system; a converging optical system for converging theharmonic wave; and holder for holding a workpiece at a position wherethe harmonic wave converged by said converging optical system isapplied.